All business is people business ultimately, and so improving your ability to size someone up should be a relentless priority — it is for me, anyway. By “size a person up” I mean figuring out how much you trust a person, how you can best collaborate with him, whether you’d hire her, whether you should fire him.

One of the simple ways I size a person up is by understanding how they understand and judge other people. In this way, I start to be build a model of the person I’m getting to know. I get to know their likes and dislikes, their biases, their underlying motivations, and of course their meta ability to evaluate people — all by hearing them talk about friends I know well.

Practically speaking, when I meet someone new, I like to ask them about someone we know in common. “So how do you know Jane?” Sure, it’s a trite question. But it can lead to a substantive exchange. It doesn’t have to be gossip. How has this person partnered with Jane? What’s frustrated him about Jane? What have been the delights?

When you ask someone to talk about their relationship with someone else, they often inadvertently reveal a lot about who they are.

At a breakfast meeting, I once asked an acquaintance — who I was also evaluating as a prospective business partner — to describe how he knew a mutual friend. As I probed, I realized this acquaintance spoke in condescending, patriarchal terms about a person who I very much considered his peer. It was revealing. I may not have gotten a glimpse at this element of his oversized ego if we had not gone down this path.

In another case, by talking about mutual friends I realized the person I was speaking to grasped subtleties about a friend’s personality that I had missed, and it made me all the more excited about partnering with him because of his extraordinary ability to make sense of at least one complicated person — and likely many others.

Bottom Line: Get to know someone new by asking him or her about someone you already know well.